Human Dlk-1 (delta-like 1 homolog (Drosophila); which may be hereinafter referred to as “hDlk-1”) is a type I transmembrane (one-transmembrane-type) protein with a full length of 383 amino acid residues which has 6 EGF-like motifs in its extracellular region. The extracellular region shows homology with a Notch/Delta/Serrate family. A hDlk-1 gene has been cloned as a molecule expressed in a GRP (gastrin releasing peptide)-responsive lung small cell carcinoma-derived cell line (Non-Patent Document 1), or as a factor for suppressing preadipocyte differentiation (Non-Patent Document 2). From the viewpoint of the homology of the amino acid sequence of hDlk-1 with that of Delta that is a ligand of a Notch receptor as a cell differentiation regulator, such Dlk-1 is generally referred to as a gene symbol, DLK1. It also has several other gene symbols such as Pref-1 (Non-Patent Document 2), pG2 (Non-Patent Document 3), SCP-1 (Non-Patent Document 4) and ZOG (Non-Patent Document 5). However, these gene symbols basically indicate the same molecule.
Moreover, hDlk-1 is cleaved with an unidentified protease which cuts the neighborhood of cell membrane in the extracellular region of hDlk-1 and it is then secreted into blood. Free hDlk-1 (hDlk-1 extracellular region) is a molecule identical to a glycoprotein called FA-1 (Fetal antigen-1) (Non-Patent Document 6) consisting of 225 to 262 amino acid residues.
The hDlk-1 gene and a gene product thereof are expressed at a high level in undifferentiated, highly proliferative, fetal cells. In particular, the hDlk-1 gene and the gene product thereof are highly expressed in fetal liver, fetal kidney, fetal skeletal muscle, fetal brain and the like. After birth, however, expression of such a hDlk-1 gene and a gene product thereof cannot be observed in most of the tissues. In normal adult tissues, the hDlk-1 gene and the gene product thereof are localized in adrenal gland, placenta and hypophysis (Patent Document 1, Non-Patent Document 2).
Furthermore, even in mature tissues, expression of hDlk-1 is observed in cells that are considered to be undifferentiated stem cells or precursor cells. For example, it has been reported that expression of hDlk-1 has been observed in hepatic oval cells that are undifferentiated and have pluripotency in adult liver (Non-Patent Documents 7 and 8), in mesenchymal stem cells that are the stem cells of bone/cartilage/adipose cells (Non-Patent Document 9), and in prostatic epithelial precursor cells in the basal cell layer of the prostate (Non-Patent Document 18). Further, it has also been reported that, in the case of mouse mesenchymal stem cells, free Dlk-1 (mouse Dlk-1 extracellular region) activates ERK/MAP kinase and induces expression of Sox-9, so that differentiation of the cells into adipose cells can be suppressed and at the same time, differentiation of the cells into chondrocytes can be induced, but that such free Dlk-1 suppresses differentiation of the cells into osteoblasts and maturation of chondrocytes (Non-Patent Documents 19 and 20). It has been suggested that hDlk-1 is associated with the properties of such tissue stem cells, such as the maintenance of undifferentiation ability.
Such an expression pattern of hDlk-1 restricted in fetal cells or stem cells and a family of genes/gene products having EGF-like motifs (Notch-receptor, Notch ligand (Delta, Jagged, serrate), etc.) generally controls the growth or differentiation of cells by intercellular interaction via EGF-like motifs. Thus, it has been suggested that hDlk-1 also has such functions. In fact, it has been well known that expression of hDlk-1 is decreased concomitant with differentiation of adipose precursor cells and that adipose differentiation is suppressed, if the hDlk-1 gene is forced to express in adipose precursor cells (Non-Patent Document 2). However, at the present time, details regarding a molecule (a ligand) interacting with hDlk-1 are unknown.
On the other hand, it has been reported that the hDlk-1 gene and the gene product thereof are expressed with a high frequency in various types of cancers or tumors. The types of cancers, in which expression of hDlk-1 has been confirmed so far, include: solid cancers such as neuroendocrine tumor, neuroblastoma, glioma, neurofibromatosis type 1, small cell lung cancer, liver cancer, kidney cancer, ovarian cancer, colon cancer, breast cancer, and pancreatic cancer (Patent Documents 1, 2, 4 and 5 and Non-Patent Documents 1, 3, 10, 11, 12, 13, 14 and 21); and blood cancers such as myelodysplastic syndrome (Patent Document 3 and Non-Patent Documents 15 and 16) and acute myelocytic leukemia (Non-Patent Document 16). It has been reported that cell growth is accelerated if a hDlk-1 gene is introduced into a K562 cell that is an erythroleukemia cell line (Non-Patent Document 16) and also that, if such a hDlk-1 gene is introduced into glioblastomas, it causes the disappearance of contact inhibition of cells as well as acceleration of cell growth, so that anchorage-independent cell growth ability can be achieved. The relationship between hDlk-1 and carcinogenesis has been suggested (Non-Patent Document 17).
Conventionally, as anti-hDlk-1 monoclonal antibodies showing cytotoxicity on human liver cancer cells in vitro in the presence of complement, rat anti-hDlk-1 monoclonal antibodies 1C1, 4C4 and 31C4 (clone names) have been known (Patent Document 1). On the other hand, these clone antibodies have also been known as antibodies that do not show anti-tumor activity (tumor growth-inhibiting activity) in vivo (in treatment models with human cancer cell-bearing mice) (Patent Documents 4 and 5).    Patent Document 1: WO 2005/052156    Patent Document 2: WO 02/081625    Patent Document 3: Japanese Patent Laid-Open No. 2001-269174    Patent Document 4: WO 2008/056833    Patent Document 5: WO 2009/116670    Non-Patent Document 1: Laborda, J. et al., J. Biol. Chem., vol. 268 (6), pp. 3817-3820 (1993)    Non-Patent Document 2: Smas, C. M. et al., Cell, vol. 73 (4), pp. 725-734 (1993)    Non-Patent Document 3: Helman, L. J. et al., Proc. Natl. Acad. Sci. USA, vol. 84, pp. 2336-2339 (1987)    Non-Patent Document 4: Maruyama, K. et al., Unpublished, Genebank accession number D16847 (1993)    Non-Patent Document 5: Halder, S. K. et al., Endocrinology, vol. 139, pp. 3316-3328 (1998)    Non-Patent Document 6: Fay, T. N. et al., Eur. J. Obstet. Gynecol. Reprod. Biol., vol. 29, pp. 73-85 (1988)    Non-Patent Document 7: Tanimizu, N. et al., Gene Expression Patterns, vol. 5, pp. 209-218 (2004)    Non-Patent Document 8: Jensen, C H. et al., Am. J. Pathol., vol. 164 (4), pp. 1347-1359 (2004)    Non-Patent Document 9: Abdallah, B. M. et al., J. Bone Miner. Res., vol. 19 (5), pp. 841-852 (2004)    Non-Patent Document 10: Jensen, C. H. et al., Br. J. Dermatol., vol. 140 (6), pp. 1054-1059 (1999)    Non-Patent Document 11: Jensen, C. H. et al., Tumour Biol., vol. 20 (5), pp. 256-262 (1999)    Non-Patent Document 12: Yin, D. et al., Int. J. Oncol., vol. 24 (4), pp. 1011-1015 (2004)    Non-Patent Document 13: Yin, D. et al., Oncogene, vol. 25 (13), pp. 1852-1861 (2006)    Non-Patent Document 14: Fukuzawa, R. et al., J. Clin. Pathol., vol. 58, pp. 145-150 (2006)    Non-Patent Document 15: Miyazato, A. et al., Blood, vol. 98, pp. 422-427 (2001)    Non-Patent Document 16: Sakajiri, S. et al., Leukemia, vol. 19 (8), pp. 1404-1410 (2005)    Non-Patent Document 17: Yin, D. et al., Oncogene, vol. 25 (13), pp. 1852-1861 (2006)    Non-Patent Document 18: Ceder, J. A. et al., Eur. Urol., Vol. 54(6), pp. 1344-1353 (2008)    Non-Patent Document 19: Sul, H S., Mol. Endocrinol., Vol. 23 (11), pp. 1717-1725 (2009)    Non-Patent Document 20: Wang, Y. et al., Mol. Cell Biol., Vol. 30(14), pp. 3480-3492 (2010)    Non-Patent Document 21: Yanai, H. et al., J. Biochem., Vol. 148(1), pp. 85-92 (2010)